Avoid using Modulepath in Javadoc plugin

[mp911de]

Java 23

Error while generating Javadoc: 
[ERROR] Exit code: 1
[ERROR] /Users/mark/.m2/repository/io/reactivex/rxjava3/rxjava/3.1.10/rxjava-3.1.10.jar(/io/reactivex/rxjava3/core/Single.class): error: Cannot attach type annotations @io.reactivex.rxjava3.annotations.NonNull,@io.reactivex.rxjava3.annotations.NonNull,@io.reactivex.rxjava3.annotations.NonNull,@io.reactivex.rxjava3.annotations.NonNull to Single.concat:
[ERROR]   class file for org.reactivestreams.Publisher not found
[ERROR] /Users/mark/.m2/repository/io/reactivex/rxjava3/rxjava/3.1.10/rxjava-3.1.10.jar(/io/reactivex/rxjava3/core/Flowable.class): error: Cannot attach type annotations @io.reactivex.rxjava3.annotations.NonNull to Flowable.blockingSubscribe:
[ERROR]   class file for org.reactivestreams.Subscriber not found
[ERROR] 2 errors
[ERROR] Command line was: javadoc -J-Duser.language= -J-Duser.country= @options @packages

Java 22

 error: class file for org.reactivestreams.Publisher not found
[ERROR] 1 error
[ERROR] Command line was: javadoc -J-Duser.language= -J-Duser.country= @options @packages

Java 21

[WARNING] Javadoc Warnings
[WARNING] warning: The code being documented uses modules but the packages defined in https://reactivex.io/RxJava/3.x/javadoc/ are in the unnamed module.
[WARNING] 1 warning

This error happens when a dependency defines a module descriptor but their transitive dependency does not. Using Javadoc legacyMode helps to resort to classpath mode to overcome this utterly broken library state.

[sbrannen]

Using Javadoc legacyMode helps to resort to classpath mode to overcome this utterly broken library state.

I'm not familiar with that legacyMode, but the JDK team introduced a --link-modularity-mismatch option for javadoc in JDK 18 indirectly per my request for such a feature.

• Reinstate links to JUnit 5 Javadoc APIs spring-framework#27497

https://github.com/spring-projects/spring-framework/blob/094e653746b69f3d4ada4bfd9f5e03744d7c6576/framework-api/framework-api.gradle#L41-L43

I see you have already closed this issue, but perhaps that -link-modularity-mismatch info option will help to address modularity issues with javadoc.